Retroreflective articles employing an outer layer of microbeads are known in the prior art. In one method of manufacture, a bead-containing coating is applied over the outer surface of the article (which may be, for example, protective helmet). The coating is allowed to dry, and is then etched to expose the microbeads. Later, a transparent sheet of vinyl chloride or other polymeric material is molded into the same shape as the article, and mounted over the outside surface, advantageously encapsulating them while leaving a small air space between the transparent material and the tops of the microbeads. Such encapsulation both protects the beads and prevents direct contact between the beads and ambient moisture, which could interfere with their retroreflective properties.
In another method of manufacture, a flexible, microbead-containing sheet material is applied over the outer surface of the article. Such sheet materials are known which have a sufficient amount of stretch to allow them to be evenly adhered over the surfaces of embossed license plates, road cones and road markers. In still another method, a microbead containing, non-woven fabric is used to make retroreflective vests and other garments for highway workers working at night.
Unfortunately, none of the known manufacturing techniques is believed to be capable of easily and quickly producing a rigid three-dimensional article having an encapsulated bead outer surface capable of high intensity retroreflection (defined in this application as a reflectivity of between about 200 to about 300 lux) without further manufacturing or assembly steps. The application and processing of a bead-containing coating over the surface of such an article is relatively cumbersome and time consuming Moreover, it is difficult to obtain the kind of high-density arrangement of the microbeads necessary to achieve high retroreflectivities of about 200 lux or more due to the irregular heights and distribution of the beads in the bonding material. Finally, such a method requires the separate fabrication and application of a protective transparent sheet material over the bead layer to encapsulate the beads.
While the application of a flexible, bead-containing retroreflective sheet material over the surface of the article may be practical for articles having simple shapes (such as license plates, road cones and other generally planar items), and does allow for a uniform and high density arrangement of microbeads, it is clearly more difficult to uniformly apply to tightly rounded and complex shapes, such as protective helmets with reinforcing ridges, protective padding or guards (e.g. knee pads, shine guards) or highly contoured shapes, such as automotive bumpers, body panels, grills and mirror housings. Of course, such a method might be modified such that only strips of retroreflective were applied to the article. However, such a modification would compromise the visibility and total reflectivity of the surface. Additionally, while many such flexible retroreflective sheet materials include a protective coating of transparent material over the bead layer, they do not include an encapsulating protective transparent sheet material that provides an air space over the bead layer. The use of such an “enclosed bead structure” in such sheet materials undesirably limits the retroreflectivity of the article to about 180 lux or less.
Clearly, there is a need for method for producing a self-supporting three dimensional article having a high-intensity retroreflective surface which is easier and less time consuming than the aforementioned prior art methods. Ideally, such a method would be easily applicable to virtually any shape, and would allow the microbeads to be arranged over the entire outer surface of the article in a high-density configuration to maximize retroreflectivity. Finally, such a method would provide a transparent sheet material over the layer of microbeads to provide the advantages of encapsulation without the need for additional fabricating steps.